The invention relates to an arrangement for printing the front or the back of a recording medium in a printer or copier which operates according to the transfer printing principle and to a method for operating an arrangement of this kind.
Printers or copiers for simplex or duplex printing are disclosed for example in U.S. Pat. No. 4,477,176 and U.S. Pat. No. 4,537,493. These are electrophotographic printing devices with a photoconductor on which at least two charge images can be arranged one behind the other, the said printing devices also having a transfer printing station with associated single-sheet positioning device (turning device) which permits the electrophotographic printing device to be operated in two operating modes, specifically, in a first operating mode in which the inked-in toner images which are arranged one behind the other on the charge image carrier are arranged one on top of the other or next to one another on one side of the single sheet, and in a second operating mode in which the sequence of toner images located on the charge image carrier are arranged on the front and the back of the single sheet. The fusing of the toner images on the single sheets is carried out with the aid of a roller fusing station using pressure and high temperatures.
In the duplex printing operating mode in which one toner image is arranged on each side of the single sheet, the fusing of the front and back takes place simultaneously. For this purpose, the single sheet which is printed on one side must be turned and printed again on the other side, and it is subsequently conveyed in a contactless fashion to the roller fusing station, for example by means of an air cushion.
This requires a very high degree of mechanical expenditure if a high degree of operating reliability is to be ensured over a large range of print matter.
It is to be generally noted that any braking, accelerating and turning device in the printer or copier exerts forces on the recording medium which make exact positioning of the recording medium, and thus reliable guidance of the recording medium in the copier or printer, more difficult. This increases the risk of blockages considerably, which can lead to frequent equipment failures. Turning, before fusing, a recording medium provided with toner images requires a considerable degree of expenditure control in order to prevent the toner images becoming blurred.
Furthermore, U.S. Pat. No. 2,990,278 and GB-A-2,040,226 disclose electrophotographic printing devices in which character-dependent charge images are produced on a photoconductor with the aid of an exposure device and are fed to a developer station. The developed charge image is then mechanically lifted off from the photoconductor using pressure with the aid of a ribbon-shaped transfer element and transmitted to a recording medium. In order to be able to fuse the toner image on the recording medium, the toner image is heated on the ribbon-shaped transfer element with the aid of a heating device and the heated toner image is applied to the recording medium by means of a roller arrangement using pressure and heat. After the transfer of the toner image onto the recording medium, the ribbon-shaped transfer element is cleaned of adhering toner in a cleaning station.
WO 82/00723 discloses an arrangement and a method for printing a recording medium on both sides. In this process, a charge image is generated on a photoconductor and then, with the aid of this charge image, an electrostatic charge image is produced on a dielectric roller or drum by ionography. It is also possible to form the charge image directly on the dielectric roller by means of an ion generator. After development with the aid of a developer station by applying a single layer of toner of a single-component developer, the single-layer toner image which is associated with the back of the recording medium is transferred, using pressure only and without heat, onto a recording roller arranged underneath the dielectric roller. Afterwards, in the same way, a toner image which is associated with the front of the recording medium is formed on the dielectric roller and then, again using pressure only and without heat the two toner images are simultaneously transferred to the front and back of the recording medium.